Cockayne syndrome (CS) is a progressive childhood neurodegenerative disorder associated with a DNA repair defect. Two genes, CSA and CSB, are specifically involved in the CS disorder. These genes are involved in nucleotide excision repair (NER) of ultraviolet damage (UV) in transcriptionally active genes (transcription coupled repair, TCP). Cs-a and Cs-b mice have much milder neurological symptoms than human patients, but a greater risk for cancer that is not usually evident in humans. We have found that crossing Cs-b mice with Xp-c mice, that are defective in NER of nontranscribed regions of the genome, increases the severity and reduces the age of onset of neurodegeneration in animals that are homozygous or heterozygous in both genes but without necessarily compromising UV sensitivity. This has resulted in mouse strains that reflect the range of severity of human CS patients and can be used as models of neurodegeneration that we will compare in detail with the human syndrome. Not all the symptoms of CS patients are however easily related to repair deficiencies, so we hypothesize that there are additional pathways relevant to the disease particularly those that are downstream consequences of a common defect in ubiquitin ligase associated with the CSA and CSB gene products. We have found that the CSB defect results in altered expression of cell cycle and anti-angiogenic genes and proteins, and more programmed cell death that are relevant to the impaired development and progressive neurodegeneration. We therefore propose that, in Aim I, we will determine whether the mouse Cs-b x Xp-c crosses recapitulate the pathology of the human disease. We will determine the specific sites of programmed cell death and whether Purkinje cell loss is a primary event or due to loss of progenitor or associated cell types. We will determine whether neurodegeneration is consistent with premature cell cycle entry and apoptosis from chronic oxidative injury. In Aim II, we will examine global and transcription coupled repair in human CS cells and mouse fibroblasts from our mouse strains and differentiation-associated repair in mouse cells of neuronal origin following damage from reactive oxygen, to identify the contributions of these repair genes to neurodegeneration. In Aim III we will determine the roles played by protein targets of CS-dependent ubiquitylation that we have identified, especially those whose over-expression may have pathological consequences. We will emphasize those targets previously identified, such as p21 and collagen 15a1, for their roles in development and neurodegeneration. Successful conclusion of these studies will expand our knowledge of mechanisms of neurodegeneration and lay groundwork for development of therapeutic approaches for CS patients.